Process for producing a strip-shaped multi-core superconductor with high Tc superconducting material and superconductor produced by this process.
The invention concerns a process for producing a strip-shaped superconductor with multiple conductor cores made of a superconducting material with a metal-oxide high Tc phase and surrounded by a normal conducting material, in which process a fabricated conductor is produced from the powdered precursor material of the superconducting material surrounded by the normal conducting material and this fabricated conductor is subjected to a cross-section reducing and precursor-material compacting deformation process and to at least one annealing treatment, where the deformation process includes at least one rolling step for flattening a conductor blank from the fabricated conductor. The invention further concerns a superconductor produced by this process. A corresponding production process and a superconductor produced by it can be seen for example in the publication xe2x80x9cPhysica Cxe2x80x9d Vol. 250, 1995, pps 340-348.
Superconducting metal oxide compounds with high transition temperatures Tc of more than 77xc2x0 K are known, which are also described as high Tc superconductor materials or HTS materials and in particular permit a liquid nitrogen (LN2) cooling technique. Among such metal oxide compounds are in particular cuprates with special structures for instance especially rare-earth-containing basic types Yxe2x80x94Baxe2x80x94Cuxe2x80x94O or rare-earth-free basic types Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O or (Bi, Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O. Within individual structures such as Bi-cuprates several superconducting high Tc phases can occur, which vary depending on the number of the copper-oxygen network planes or layers within the individual crystalline cells and the different transition temperatures Tc.
Attempts have been made using known HTS materials to produce long lengths of superconductor in wire or strip shape. A procedure known to be suitable for this purpose is the so-called xe2x80x9cpowder-in-tube techniquexe2x80x9d which is known principally from the production of superconductors with the classic metallic superconducting material Nb3Sn. A similar technique for producing conductors from HTS materials in a tubular sheath or in a matrix of normal conducting material, particularly silver or silver alloy, introduces a normally powder precursor of the HTS material that in general contains as yet none or only a small fraction of the desired superconducting high Tc phase. The fabricated conductor thus obtained is then brought to its desired final dimensions through various deformation processes that can be interrupted as desired by at least one high-temperature heat treatment. In order to achieve or optimize the superconducting properties or to create the desired high Tc phase, the semi-finished conductor thus obtained subsequently undergoes at least one annealing treatment which can be interrupted as desired by a further deformation process.
Combining in conventional fashion high Tc superconductors or their corresponding fabricated conductors or semi-finished conductors enables the creation of conductors with multiple superconducting conductor cores, so-called multi-core or multi-filament superconductors.
Known multi-core superconductors made of HTS material are preferably strip-shaped. Producing a finished-product conductor in this form requires a rolling process according to the literature cited above. Prior to this rolling process, however, the fabricated conductor has to be transformed into a generally cylindrical pre-deformed and pre-compacted compound body with a generally even cross-sectional distribution of conductor cores. This compound body hereinafter referred to as the conductor blank is then transformed by means of a rolling process generally incorporating several rolling steps into the flat strip shape so as to provide the necessary texture for a high current carrying capacity, that is an essentially parallel alignment of the crystalline planes of the superconducting phase. For this purpose the superconductor precursor material must be compacted to the greatest possible extent especially during deformation of the conductor blank through the rolling process.
It is evident, however, that such a process for producing a strip-shaped multi-core superconductor leads to an uneven distribution of individual conductor cores seen in overall cross-sectional perspective. The individual conductor cores here vary in thickness and in width and the differing degrees of compacting of their powder precursor material lead to an uneven distribution of current in the finished-product conductor. The primary cause of this unevenness is the minimum of one rolling step, in which conventionally a pair of rollers is separated by a gap with a rectangular rolling region. Here the central regions of the conductor are pressed especially hard, while the lateral regions experience scarcely any compacting of the precursor material. Thus in the finished-product conductor the central conductor cores carry a higher current than the outer (lateral) conductor cores.
Efforts have been made to overcome this problem by designing in advance a fabricated conductor in rectangular shape (see for example EP 0 509 436 A). The cost of constructing and deforming such a shape of conductor, however, is very high.
The task of the present invention is to design a process with the characteristics initially described so as to enable the production of a strip-shaped multi-core superconductor which particularly in its lateral edge regions possesses an improved current bearing capacity (or critical current density) in relation to conventional designs.
This task is fulfilled according to the present invention in that during at least one rolling step the conductor blank is fed into a gap between two rollers of which the rolling surfaces describe a concave, at least approximately elliptical, contour at least in the rolling region. Here the deviation from the ideal form of an ellipse should be at most large enough that the contour lies within a region that can be described by two concentric ellipses where the outer ellipse possesses a primary axis and a secondary axis that are each about 10% larger than the corresponding axes of the enclosed inner ellipse.
The advantages obtained with this version of the process can be seen especially in that the elliptical contour of the rolling surfaces, also known as the rolling track, provides a more regular alignment of the individual conductor cores and a more extensive broadening of the lateral edge conductors. For while the central region of the conductor blank is rolled to a conventional thickness in standard fashion by means of the elliptical rolling geometry, the lateral edge regions are deformed more strongly to a smaller thickness. Since however the number of conductor cores is lower in the lateral region, the result is a relatively regular geometry. The higher compacting of the conductor cores yields an advantageously higher overall critical current density for the finished-product conductor. This form of the rolling process according to the invention also has a positive influence upon the so-called aspect ratio, that is on the quotient of thickness to width, of the multi-core conductor.
A particularly advantageous arrangement is when during several rolling steps the minimum two roller gaps vary the elliptical contour of their opposing rolling surfaces through the ratio of primary elliptical axis to secondary elliptical axis. This arrangement permits the contour to be adjusted with each rolling step to fit the increasing broadening of the conductor blank.
The strip-shaped multi-core superconductor produced according to the invention thus has to its advantage an at least approximately elliptical cross-section. The permissible deviation from an elliptical shape depends on the contour of the rolling surfaces.
Further advantageous versions of the process according to the invention and of the multi-core superconductor produced by it arise from the respective claims.